Production of ceramic-grade metal oxide powders containing mixed values of heavy metals has been a formidable problem for industrial and nuclear fuel operators handling aqueous nitrate solutions in their processes. It has particularly been a problem in preparing nuclear fuel for fast neutronic reactors utilizing uranium, thorium, plutonium, and their combinations with each other or with rare earth elements, such as cerium. Mixed nuclear fuels are desirable as a means to reduce the threat of proliferation of nuclear weapons.
If stolen or otherwise diverted, significant upgrading of these fuels would be necessary to obtain weapons-grade nuclear material since it is diluted or denatured with non-fissionable materials or rare earth elements. Typically, mixed nuclear fuels have been coprocessed by precipitation and thermal decomposition processes. Alternatively, they have been prepared in similar processes in isolated form followed by blending of the product powders in a final mixed fuels fabrication step. This alternative process is objectionable because diversion prone purified metal solutions or oxides are present in the processes in isolated form. While precipitation and decomposition processes can operate with denatured fuel mixtures, these coprocessing technologies frequently involve solids handling operations which are messy, dust prone, and cost or energy intensive. The most objectionable aspects of these operations have the dust generation caused by slurry transfers of precipitates, powder pretreatment for pellet fabrication, and dimensional correction of the fabricated pellet. Because of the radioactivity and toxicity of nuclear fuels of the actinide series, these operations must be carried out in remotely operable facilities which are not amenable to dusty processes. Additionally, materials accountability and criticality control are seriously hampered in such processes. Therefore, the economic and commercial feasibility of large-scale nuclear fuel coprocessing and recycling facilities would be significantly enhanced by the elimination or reduction of dust-generating procedures.
Ceramic pellets for nuclear fuel applications must meet rigid specifications. It has been difficult to consistently produce pellets from decomposition or precipitation processes which meet all of the requisite properties for fuel quality pellets. While these processes can individually produce some of these properties, they cannot produce all of them in combination. Often, the oxides of these processes must be modified with binders, additives, lubricants, or pore formers to produce acceptable compacts. However, these additives have constituents which are not suitable for reactor exposure. Thus, many fabricated pellets are still rejected after extensive costs are incurred for chemical additives, specialized preparation, or sizing procedures. These rejections are usually based on non-attainment of a satisfactory combination of the following properties: uniform composition, dimensional precision, high sintered density, good mechanical strength, and high thermal conductivity. Without such properties, an irradiated fuel pellet cannot withstand mechanical and thermal impact or stresses to which it may be subjected during its service life. Undesirable side effects may be distortion, bloating, spalling, and release of residuals which could cause fuel rod failure and subsequent contamination of the reactor cooling system.
The novelty of the present invention lies in the discovery of a conversion process utilizing thermal decomposition of concentrated metal nitrates to produce powders possessing precipitation-like characteristics for fabrication of fuel quality pellets. This is accomplished by the simple and inexpensive addition of ammonium nitrate to the heavy metal solutions followed by thermal decomposition in the temperature range of about 300.degree. to 800.degree. C. An unexpected advantage of such addition is the derivation of good quality ceramic powders without actual precipitation or the associated increase in production and auxiliary equipment to handle, monitor, and control coprecipitation technology and its wastes.